In certain types of thermal printers, a receiver of print medium, such as paper, and a dye-donor film are moved past a print head as the print head causes an image to be transferred to the receiver. The receiver is moved past the print head in a series of repetitive passes. Each pass is made using a different color dye-donor film. In this manner, a series of overlying colored images are generated on the receiver. When the overlying images are properly registered with one another, the resultant image on the receiver is a full color image.
The printhead in this the of printer is constructed as a strip of discrete resistor elements. The strip of resistor elements is long enough to span an entire width of a receiver. Each resistor element can be activated independently. When a resistor element is activated, or "on", heat from the element produces a transfer of a spot of dye from the dye-donor film to the receiver.
As the receiver is transported across the printhead an image is formed as a series of lines. Each line of the image is unite. Each line is formed by a unique combination of the resistor elements being on or off.
In order to produce high resolution color images, it is necessary to produce very small dots of the transferred dye. This requires that the resistor elements be correspondingly small and very closely spaced on the printhead. In a typical thermal printer, a printhead is six to eight inches long and is comprised of thousands of resistor elements.
As each resistor element is turned on, some of the energy delivered to the element is transferred to the dye. However, a large portion of the energy remains in the printhead as residual energy. With continued operation of the printhead, this residual energy accumulates and causes a gradual overall heating of the printhead. As the ambient temperature of the printhead changes, its printing characteristics change. A printhead which is excessively hot, for example, will produce images which are smeared and unclear.
Thermal printers are designed to control the temperature of a printhead thereof. A typical prior art technique of controlling the temperature of a printhead makes use of a single feedback temperature device such as a thermocouple or thermistor in conjunction with a cooling system that consists of cooling fans and a heat sink on the back of the printhead. If the temperature of the printhead reaches a predetermined level, the cooling fans are turned on in an attempt to maintain the printhead/heat sink at a preselected temperature. However, although the thermal diffusion of the printhead/heat sink is generally fast, the ability of the fans to maintain true uniform control is limited. One example of an arrangement which uses cooling fins is disclosed in U.S. Pat. No. 4,896,166 (Barker et al.), issued Jan. 23, 1990.
There have been suggested a variety of other techniques to control temperature of printheads which have employed elaborate cooling systems. For example, a thermal printer disclosed in U.S. Pat. No. 4,819,011 (Y. Yokota), issued Apr. 4, 1989, employs a heat pipe and a thermoelectric transducer to maintain temperature control of a printhead. Such an elaborate system is not practical in thermal printers which are designed for use in typical office settings. A thermal printer for an office setting is designed to be used in conjunction with a personal computer as an adjunct or a substitute for a laser printer. In this context, the thermal printer must be marketed at a relatively low price. Consequently, the manufacturing cost of such a thermal printer must be held to a very low level, in the order of a few hundred dollars. Elaborate heat pipe and thermoelectric transducer temperature control systems are not economically practical in these office-setting thermal printers.
It is desirable therefore to produce a low cost thermal printer with a printhead/heat sink that can be maintained at essentially a uniform temperature throughout irrespective of the usage profile of the printer.